In high voltage applications bushings are used for insulating a high voltage conductor when passing the conductor through a conductive structure having a different potential than the high voltage conductor. Bushings are for instance utilized in inductive devices such as transformers and reactors in order to be able to provide a high voltage conductor through the grounded housing of the device.
High Voltage Direct Current (HVDC) transformers are due to their connection to the DC grid subjected to both AC and DC voltage components. Therefore, such transformers need to be designed taking both AC and DC voltage stress considerations into account.
WO2007/111564 discloses a high voltage insulating system for HVDC and a method for manufacturing such an insulating system. WO2007/111564 discloses a bushing arranged in a turret of a transformer, a bushing conductor extending through the bushing, a connection between the bushing conductor and the transformer conductor extending from the transformer tank, and an insulating system comprising inter alia a cylindrical insulating barrier and a transformer insulating material enclosing the connection.
The arrangement of WO2007/111564 consumes quite a large amount of space in the turret if it is to fulfill the safety provisions concerning the handling of very high voltages.
JP60193313 discloses a direct current electric apparatus filled with oil. A DC transformer having a core around which a coil is wound. The DC transformer is filled with oil and has a bushing. At the bottom of the bushing, an insulation shield is provided around the bushing terminal. The bushing terminal and the coil are connected with an insulated high voltage lead wire consisting of a lead wire with an insulation layer.
EP0795877 discloses a DC bushing having an increased DC dielectric strength and includes a porcelain tube constituting a lower portion thereof and immersed in an insulating oil contained in a tank, a lower insulated shield provided at a lower end portion of the porcelain tube and composed of a shielding electrode covered with an insulating cover, and a shield barrier disposed around an outer periphery of the lower insulated shield with an oil gap being defined between the shield barrier and the lower insulated shield. The thickness of the shield barrier is greater than a width of the oil gap as viewed in a direction widthwise thereof.
Hence, there is a need to improve existing insulating solutions for high voltage applications in general, and for HVDC applications in particular.